This section provides background information related to the present disclosure which is not necessarily prior art.
Modern windows in residential, commercial and industrial buildings may include one or more window sashes that are movable within a window jamb. Hung windows include window sashes that move vertically to open and close and often include two or more window balance assemblies. The balance assemblies bias the window sash upward (i.e., toward an open position for a lower sash or toward a closed position for an upper sash) to assist a user in moving the window sash and to retain the window sash at a position selected by the user.
Moving coil, constant-force window balance assemblies for hung windows are known. Such a window balance assembly can include a curl spring which produces a constant force or tension of a recommended or rated value as it is linearly extended (i.e., uncurled) and retracted (i.e., curled). The spring force correlates to a weight range of window sashes that can be accommodated by the window balance assembly.
However, in a moving coil, constant force window balance assembly (such as shown in FIG. 2A), when the curl spring is retracted less than a minimum retracted length, the curl spring may not produce a spring force at least equal to the rated constant force value. See FIGS. 2B and 2C. Consequently, if movement of a window sash causes the curl spring to retract less than its minimum retracted length (e.g., a lower sash fully-opened or an upper sash fully-closed), the window balance assembly may be incapable of producing a counter-balance or holding force sufficient to retain the window sash in that position.